1. Field of the Invention
This invention relates to a method of judging whether a limited irradiation field is present or absent on a recording medium such as a stimulable phosphor sheet in the case where a radiation image has been recorded on the recording medium. This invention also relates to a method of selecting a correct irradiation field wherein a correct prospective irradiation field region is selected from a plurality of prospective irradiation field regions in the case where they are present on a recording medium. This invention further relates to a method of judging correctness or incorrectness of an irradiation field in which judgment is made as to whether a prospective irradiation field region is correct or is not correct in the case where at least one prospective irradiation field region is present on a recording medium.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .lambda.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and then is exposed to stimulating rays such as a laser beam which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is detected photoelectrically and converted to electric image signals, image processing is carried out on the electric image signals, and the radiation image of the object is reproduced as a visible image by use of the processed image signals on a recording material such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing system, in order to improve the image quality, and particularly the diagnostic efficiency and accuracy, of the visible image, image read-out for photoelectrically detecting the light emitted by the stimulable phosphor sheet upon stimulation thereof should be carried out preferably by use of read-out conditions adjusted to appropriate values in accordance with each radiation image, and/or image processing of the image signals preferably should be carried out by use of optimal image processing conditions adjusted in accordance with each radiation image.
One embodiment of the method of adjusting the read-out conditions and/or the image processing conditions such as gradation processing conditions in accordance with each radiation image has been proposed in, for example, Japanese Unexamined Patent Publication No. 58(1983)-67240 whose U.S. equivalent is U.S. Pat. No. 4,527,060. In the proposed embodiment, before final read-out is carried out by scanning the stimulable phosphor sheet carrying a radiation image of an object stored thereon by stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the radiation energy stored, detecting the emitted light by a photoelectric read-out means and converting it into electric image signals, preliminary read-out for approximately detecting the image information stored on the stimulable phosphor sheet is carried out by use of stimulating rays of a level lower than the level of the stimulating rays used in the final read-out. The read-out conditions (i.e. the read-out conditions for the final read-out, such as a read-out gain and a scale factor) and/or the image processing conditions (such as gradation processing conditions and frequency response processing conditions) are adjusted on the basis of the image signals obtained by the preliminary read-out (hereinafter referred to as preliminary read-out image signals).
As the method of adjusting the read-out conditions for the final read-out on the basis of the preliminary read-out image signals, a novel method has been proposed in, for example, Japanese Unexamined Patent Publication No. 60(1985)-156055 whose U.S. equivalent is U.S. Pat. No. 4,682,028. The proposed method comprises the steps of: determining a histogram of the preliminary read-out image signals (image signal levels), calculating the maximum image signal level Smax and the minimum image signal level Smin of a desired image signal range in the histogram, and adjusting the read-out conditions for the final read-out so that the maximum image signal level Smax and the minimum image signal level Smin correspond respectively to the maximum signal level Qmax and the minimum signal level Qmin of a desired input signal range in an image processing means which are determined by the maximum density Dmax and the minimum density Dmin of a correct density range in the reproduced visible image.
Also, as the method of adjusting the image processing conditions, for example, gradation processing conditions, on the basis of the preliminary read-out image signals, the method as mentioned above may be used by way of example. Specifically, there may be used a method comprising the steps of: determining a histogram of the preliminary read-out image signals, calculating the maximum image signal level Smax and the minimum image signal level Smin of a desired image signal range in the histogram, and adjusting the gradation processing conditions so that the maximum image signal level Smax and the minimum image signal level Smin correspond respectively to the maximum signal level Rmax and the minimum signal level Rmin of a desired input signal range in an image reproduction means (visible image output means) which are determined by the maximum density Dmax and the minimum density Dmin of a correct density range in the reproduced visible image.
The image processing conditions such as the gradation processing conditions may be adjusted based on the image signals obtained by the final read-out as well as the image signals obtained by the preliminary read-out. Also, in this case, it is possible to employ, for example, the method comprising the steps of: determining a histogram of the image signals obtained by the final read-out, calculating Smax and Smin from the histogram, and adjusting the gradation processing conditions so that Smax and Smin correspond respectively to Rmax and Rmin.
On the other hand, in the case where the aforesaid radiation image recording and reproducing system is used for medical diagnosis, portions of the human body not related to diagnosis should not be exposed to radiation since the radiation is harmful to the human body. Further, when the human body portions not related to diagnosis are exposed to radiation, the radiation is scattered by such portions to the portion related to the diagnosis, and the contrast and resolution are affected adversely by the scattered radiation. Therefore, in the aforesaid radiation image recording and reproducing system, the irradiation field is often limited in the course of recording of a radiation image. However, in the case where image recording is carried out by the limitation of the irradiation field as mentioned above, as shown in FIG. 2, radiation (indicated by dots in FIG. 3) scattered by the object within an irradiation field 14 normally passes outside of a region 12 outside of the irradiation field 14 on a stimulable phosphor sheet 10. The scattered radiation is absorbed and stored on the stimulable phosphor sheet which exhibits high sensitivity, and therefore the histogram of the preliminary read-out image signals includes the image signal level caused by the scattered radiation. Since the image signal level caused by the scattered radiation outside of the irradiation field on the stimulable phosphor sheet is often higher than the image signal level inside of the irradiation field, it is not always possible to discriminate between the image signal levels inside and outside of the irradiation field in the histogram obtained by the preliminary read-out. Therefore, in the case where Smax and Smin are calculated from the histogram as mentioned above and the read-out conditions for the final read-out are adjusted on the basis of Smax and Smin, the minimum image signal level inside of the irradiation field is not detected as Smin, and that image signal level caused by the scattered radiation outside of the irradiation field is detected as Smin. In general, the minimum image signal level outside of the irradiation field is lower than that inside of the irradiation field. Accordingly, when the minimum image signal level outside of the irradiation field is detected as Smin, signals caused by the scattered radiation not related to diagnosis are taken within a low density range in the final read-out, and the density of the image of the portion related to diagnosis becomes too high. As a result, the image contrast decreases, and it becomes difficult to make an accurate diagnosis.
Namely, in the case where a radiation image is recorded by limiting the irradiation field, radiation scattered by the object passes outside of the irradiation field on the stimulable phosphor sheet, and noise caused by the scattered radiation is contained in the preliminary read-out image signals. Therefore, when the read-out conditions for the final read-out are adjusted based on such preliminary read-out image signals, it is not always possible to adjust the read-out conditions for the final read-out to appropriate values and to obtain a visible image suitable for viewing, particularly for diagnostic purposes.
The aforesaid problem arises in the case where the read-out conditions for the final read-out are adjusted based on the preliminary read-out image signals as mentioned above, and in the case where the image processing conditions such as gradation processing conditions are adjusted based on the preliminary read-out image signals or the final read-out image signals.
Therefore, in the case where the read-out conditions for the final read-out and/or the image processing conditions are to be adjusted based on the preliminary read-out image signals and/or the final read-out image signals by use of the aforesaid method, it is desired to judge whether the irradiation field was limited in the course of image recording on the stimulable phosphor sheet, i.e. whether a limited irradiation field is present or absent on the stimulable phosphor sheet. In the case where the limited irradiation field is present on the stimulable phosphor sheet, the adverse effects of scattered radiation outside of the irradiation field should be eliminated by, for example, employing a value of a level higher by a predetermined amount than the minimum image signal level in the histogram of the image signals in the course of adjustment of Smin from the histogram, or by recognizing the irradiation field by one of irradiation field recognizing methods proposed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-39039 whose U.S. equivalent is U.S. Pat. No. 4,851,678, 62(1987)-15536, and 62(1987)-15537 through 62(1987)-15541, and adjusting the read-out conditions for the final read-out and/or the image processing conditions based on only the preliminary read-out image signals and/or the final read-out image signals inside of the irradiation field.
As mentioned above, judgment as to the presence or absence of a limited irradiation field is necessary in the case where image recording has been carried out by use of the stimulable phosphor sheet and the read-out conditions for the final read-out or the like are to be adjusted. The judgment of the presence or absence of a limited irradiation field is also necessary in various other cases.